These are the small bal bearings which were specific to the Norton SOHC Pressure Release Mechanism behind the timing case.
This ball fulfills a very important role, as it controls the pressure to the SOHC oil feed system, in conjunction with the pressure release adjuster and spring. This ball sits against a tapered seat within the crankcase and has a spring behind it and the pressure release adjuster bolt - which control how easily and how much the ball can raise off its seat, reducing overall oil pressure from the maximum, via an oil bypass loop
One ball is fitted per engine (and these are a special Imperial size as per the original), but it is very easy to lose one when stripping to clean and check the mechanism - so we supply as a pack of 3 - it is always worth keeping a couple in your toolbox
Note: Read also our Item Descripton for the main oil pressure adjuster for more information - but it is always a good idea to ensure that this ball is able to fully seat on the seating in the crankcase, with no dirt or debris in the seat area - which can affect overall oiling and add to the chance of Wet Sumping.